This invention relates to transparencies made with hot melt ink and to methods for making such transparencies.
Hot melt inks are used in thermal transfer printers and in certain ink jet printers. The characteristic of these inks is that they are solid at room temperature, liquified by heating for marking, and resolidified by freezing on the marked substrate.
Transparency substrates are made of transparent sheet material, such as a polyester material, which is not receptive to liquid materials such as ink. When solvent-based inks are used to make transparencies, the substrate is coated with a layer receptive to the ink and the ink is absorbed into the coating. For example, U.S. Pat. Nos. 4,528,242 to Burwasser, 4,547,405 to Bedell et al., 4,555,437 to Panck, 4,575,465 and 4,578,285 to Viola, and 4,592,954 to Malhotra disclose special coatings which are capable of absorbing inks for transparent base material such as Mylar. Hot melt inks, however, do not penetrate into the substrate or into a coating on the substrate but adhere to the surface and retain a three-dimensional form. In this way they are distinct from inks which are absorbed or dry into a flat spot through evaporation or absorption.
When projected from a transparency, the deposited three-dimensional ink spots tend to scatter transmitted light in the manner of a dioptric lenticule. The small lenticules formed by the three-dimensional ink spots refract light which passes through them away from the path to the projection lens so that they cast gray shadows in projection irrespective of the color of the ink which forms the lenticule.
Attempts have been made to overcome this problem by flattening the three-dimensional ink spots on the transparent substrate, but such flattening affects only the uppermost portions of the spot, leaving the peripheral portions of the spots curved so as to refract most of the light passing through the ink spots away from the path to the projection lens. Consequently, although flattening of three-dimensional ink spots in a transparency may produce a slight improvement, the images made in this manner are still unsatisfactory.